Cube 2: Sauerbraten

How to: To easily boost health, select single player mode and choose an open space with no enemies at the start. Press E to enter Edit mode. Next, press [F3] to spawn a new entity (the default is shotgun shells). Now, press [F4] to change the entity to a health boost upgrade. Repeat these steps to spawn as many health boosts as desired. Each upgrade gives you an additional 5 HP to the maximum and remains after death.

*** While in Edit mode you will fly and see through walls. However, enemies will respawn when you enter and exit editing mode.